Eye lenses from young rats or mice synthesize GSH from methionine or N-acetylcysteine. However, lenses from old animals do not synthesize GSH from methionine. This is due to the absence of cystathionase activity in old lenses. GSH monoethyl ester, but not free GSH, increases GSH content and protects the lens against experimental oxidative stress. The importance of these results in the prevention of cataractogenesis is discussed.